diff --git a/nanobot/channels/matrix.py b/nanobot/channels/matrix.py
index 5893bb2..504e11f 100644
--- a/nanobot/channels/matrix.py
+++ b/nanobot/channels/matrix.py
@@ -173,9 +173,16 @@ def _render_markdown_html(text: str) -> str | None:
     return formatted
 
 
-def _build_matrix_text_content(text: str) -> dict[str, str]:
+def _build_matrix_text_content(text: str) -> dict[str, Any]:
     """Build Matrix m.text payload with plaintext fallback and optional HTML."""
-    content: dict[str, str] = {"msgtype": "m.text", "body": text}
+    content: dict[str, Any] = {
+        "msgtype": "m.text",
+        "body": text,
+        # Matrix spec recommends always including m.mentions for message
+        # semantics/interoperability, even when no mentions are present.
+        # https://spec.matrix.org/v1.17/client-server-api/#mmentions
+        "m.mentions": {},
+    }
     formatted_html = _render_markdown_html(text)
     if not formatted_html:
         return content
diff --git a/tests/test_matrix_channel.py b/tests/test_matrix_channel.py
index 6a33a5e..f55fd0f 100644
--- a/tests/test_matrix_channel.py
+++ b/tests/test_matrix_channel.py
@@ -627,7 +627,11 @@ async def test_send_clears_typing_after_send() -> None:
     )
 
     assert len(client.room_send_calls) == 1
-    assert client.room_send_calls[0]["content"] == {"msgtype": "m.text", "body": "Hi"}
+    assert client.room_send_calls[0]["content"] == {
+        "msgtype": "m.text",
+        "body": "Hi",
+        "m.mentions": {},
+    }
     assert client.typing_calls[-1] == ("!room:matrix.org", False, TYPING_NOTICE_TIMEOUT_MS)
 
 
@@ -678,6 +682,7 @@ async def test_send_adds_formatted_body_for_markdown() -> None:
     content = client.room_send_calls[0]["content"]
     assert content["msgtype"] == "m.text"
     assert content["body"] == markdown_text
+    assert content["m.mentions"] == {}
     assert content["format"] == MATRIX_HTML_FORMAT
     assert "<h1>Headline</h1>" in str(content["formatted_body"])
     assert "<table>" in str(content["formatted_body"])
@@ -698,6 +703,7 @@ async def test_send_adds_formatted_body_for_inline_url_superscript_subscript() -
     content = client.room_send_calls[0]["content"]
     assert content["msgtype"] == "m.text"
     assert content["body"] == markdown_text
+    assert content["m.mentions"] == {}
     assert content["format"] == MATRIX_HTML_FORMAT
     assert '<a href="https://example.com" rel="noopener noreferrer">' in str(
         content["formatted_body"]
@@ -764,7 +770,7 @@ async def test_send_falls_back_to_plaintext_when_markdown_render_fails(monkeypat
     )
 
     content = client.room_send_calls[0]["content"]
-    assert content == {"msgtype": "m.text", "body": markdown_text}
+    assert content == {"msgtype": "m.text", "body": markdown_text, "m.mentions": {}}
 
 
 @pytest.mark.asyncio
@@ -778,4 +784,8 @@ async def test_send_keeps_plaintext_only_for_plain_text() -> None:
         OutboundMessage(channel="matrix", chat_id="!room:matrix.org", content=text)
     )
 
-    assert client.room_send_calls[0]["content"] == {"msgtype": "m.text", "body": text}
+    assert client.room_send_calls[0]["content"] == {
+        "msgtype": "m.text",
+        "body": text,
+        "m.mentions": {},
+    }